The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation by fragmenting such formation to form a stationary, fragmented, permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of a gaseous combustion zone feed comprising oxygen downwardly into the combustion zone to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the gaseous combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the combustion zone feed downwardly into the retort, the combustion zone is advanced downwardly through the retort.
The effluent gas from the combustion zone comprises combustion gas and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon and sulfurous compounds. It passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid hydrocarbon products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and any gaseous portion of combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort to the surface.
It is desirable to know the locus of parts of the combustion and retorting processing zones as they advance through an in situ oil shale retort for many reasons. One reason is that by knowing the locus of the combustion zone, steps can be taken to control the orientation of the advancing side of the combustion zone. It is desirable to maintain a combustion zone which is flat and uniformly transverse and preferably uniformly normal to the direction of its advancement. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to oxidize hydrocarbon products produced in the retorting zone, thereby reducing hydrocarbon yield. In addition, with a skewed combustion zone, more cracking of the hydrocarbon products can result. Monitoring the locus of the combustion zone provides information for control of the advancement of the combustion zone to maintain it flat and uniformly perpendicular to the direction of its advancement to obtain high yield of hydrocarbon products.
Another reason for monitoring the locus of the combustion zone is so that the composition of the combustion zone feed can be varied with variations in the kerogen content of the oil shale being retorted. If combustion zone feed containing too high a concentration of oxygen is introduced into a region of the retort containing oil shale having a high kerogen content, oxidation of carbonaceous material in the oil shale can generate so much heat that fusion of the oil shale can result. High temperatures also can cause excessive endothermic carbonate decomposition to carbon dioxide and dilution of the off gas from the retort, thereby lowering the heating value of the off gas. Layers in the fragmented mass are correlated with strata in the unfragmented formation because there is little vertical mixing between strata when explosively fragmenting formation to form a fragmented permeable mass of formation particles. Therefore, samples of various strata through the retort can be taken before initiating retorting of the oil shale and assays can be conducted to determine the kerogen content. Then, by monitoring the locus of the combustion zone as it advances through the retort, the composition of the combustion zone feed can be appropriately modified.
Another reason for monitoring the locus of the combustion and retorting processing zones as they advance through the retort is to monitor the performance of the retort to determine if sufficient shale oil is being produced for the amount of oil shale being retorted.
Also, by monitoring the locus of the combustion and retorting zones, it is possible to control the advancement of these two zones through the retort at an optimum rate. The rate of advancement of the combustion and retorting zones through the retort can be controlled by varying the flow rate and composition of the combustion zone feed. Knowledge of the locus of the combustion and retorting zones allows optimization of the rate of advancement to produce hydrocarbon products of the lowest cost possible with cognizance of the overall yield, fixed costs, and variable costs of producing the hydrocarbon products.
Thus, it is desirable to provide a method for monitoring advancement of combustion and retorting processing zones through an in situ oil shale retort.